European Patent Applications EP-A-523,183, EP-A-520,888 and EP-A-520,889 describe small spherical particles having the advantage of being injectable. However, the nanoparticles thus prepared have mean diameters of the order of 50 to 500 nm and may not be sterilized by sterilizing filtration without a considerable loss in yield, and/or may not be freeze-dried owing to insufficient stability.
In Eur. J. Pharm. Biopharm., 39(5), 173-191 (1993) the authors examined the technologies currently available in the field of nanoparticles intended for the pharmaceutical industry. It is stated on page 182 that the sterile filtration of nanoparticle suspensions has never been described.
According to the present invention, it is possible to prepare particles, of which 95% have a mean diameter less than 100 nm, and more preferably whose mean diameter is between 20 and 75 nm and which may thus be subjected to sterile filtration on a 0.22 .mu.m filter without loss of yield. The particles are, moreover, more stable than those obtainable according to the prior art and may be freeze-dried without any agglomeration of the particles occurring.